Surge protection circuits are customary, in particular in the area of telecommunications and in high-voltage technology. Clamp circuits have been commonly used previously to protect input lines of integrated circuits, e.g., of SAE diagnostic inputs of motor vehicle controllers; such clamp circuits operate by acting upon the input signals and converting the resulting power loss into heat via resistors, for example, in Zener diodes.
Since automobile diagnostic lines leading to a diagnostic interface are laid in cable harnesses, short-circuits may occasionally occur between these lines due to attrition or the like, which may result in failure of circuit components or to faults. A diagnostic line, for example, may be short-circuited with a ground wire or with a line conducting battery voltage +U.sub.BAT. A short-circuit to ground represents no problem for the integrated input circuit of a vehicle controller, whereas a short-circuit with the battery voltage +U.sub.BAT, at least in 24-V on-board systems, results in the allowable input range of the SAE diagnostic interface IC being exceeded and consequently in the possible destruction of this integrated circuit.